Stress relaxation gage

ABSTRACT

Contemporaneously with the displacement of underground medium to form an excavation, measurements of deformation in the medium surrounding the excavation are taken by an arrangement of probe sensors in boreholes drilled in this surrounding medium. Ends of the sensors projecting from the boreholes are referenced to mount structures fixed to a stable substance, such as a distance extension of the surrounding medium, and the opposite sensor ends inside the boreholes are flexibly joined to anchors embedded at the deep ends of the boreholes. Strain gages affixed to cantilevers connected between probe structures and the flexible joints produce signals when shifting of the medium moves the anchors therein with respect to the referenced probe structures. These signals are interpreted as data on deformation during excavation, and subsequently time-dependent deformation.

United States Patent Inventors Galen Glen Weddell Spokane; Earl LeePhfllps, Newman Lake, both of, Wash. App]. No. 858,427 Filed Sept. 16,I969 Patented An. 24, I971 Assignee The Unled States ol America Irepreseatedbythesecretaryoltbe Interior S'I'RBSRELAXA'I'ION GAGEBCIahuJDrawlngl'b.

. 73/88 E. 33/1 H, 33/125 R Int. CL GOllr 7/18 Field of Search ..73/88E, 84, 151,885; 33/1 H, I25; 175/50; 299/l Helm Cited UNITED STATESPATENTS 3,416,230 12/1968 Oleson eta]. 73/88 E X 8/1969 Geary et al.33/1 3,481,189 12/1969 Brennanetal.1..1... .1/88 5 3,482,443 12/1969Nichols et air 1. .1 1. 71/151 2,599,578 6/1952 Obertet .1 W881) PrimaryExaminer-Charles A. Ruehl Assistant Examiner--Marvin SmollarAnorney.r-Ernest S. Cohen and Gersten Sadowsky ABSTRACT:contemporaneously with the displacement of underground medium to fonn anexcavation, measurements ot deformation in the medium surrounding theexcavation are taken by an arrangement of probe sensors in boreholesdrilled in this surrounding medium. Ends of the sensors projecting fromthe boreholes are referenced to mount structures fixed to a stablesubstance, such as a distance extension of the sur rounding medium, andthe opposite sensor ends inside the boreholes are flexibly joined toanchors embedded at the deep ends of the boreholes. Strain gages affixedto cantilevers connected between probe structures and the flexiblejoints produce signals when shifting of the medium moves the anchorstherein with respect to the referenced probe struc tures. These signalsare interpreted as data on defamation during excavation, andsubsequently time-dependent defor mation.

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STRESS RELAXATION GAGE BACKGROUND OF THE INVENTION The invention is animprovement in the field of rock and soil mechanics, and is particularlyapplicable to measuring deformation, including time dependentdeformation of the medium surrounding a tunnel-type underground openingas it is being excavated. Information derived thereby is useful indetermining significant properties of this medium important to the studyof its physical relationships with respect to time. Procedurespreviously applied in studying relationships between deformation rate,rock anisotropy and rock competence with time, required data determinedover extended periods of time, such as a three months sampling periodrelied on to provide the basis for solving long term differentialsupport problems. A hypothesis leading to the present invention arosefrom a concept that requisite data could be obtained in a shortenedsampling period of a day or less by measuring initial and time dependentdeformation of a full-scale tunnel during excavation.

DESCRIPTION OF THE PRIOR ART Prior attempts to take full-scalemeasurements of the deformation of the medium around a tunnel at thetime for initial deformation are unknown. However, measurementsapproximating this type of investigation have been made in an extendedtime period subsequent to excavation. An overcore stress-relief methodwhich was used applied a sensing probe by positioning it in a hole whichwas subsequently removed from the appertaining rock wall within a coreof much larger diameter. A radial deformation could be measured in thismanner with a sensing probe having as an integral part thereof sixsensor elements oriented radially therein. Measurements obtained wererelated to known rock properties to determine the in situ rock stress. Adescription in greater detail of this type of probe is available in theU. S. Bureau of Mines Report of Investigations 70l5 entitled"Three-Component Borehole Deformation Gage For Determining the Stress inRock" By R. H. Merrill, dated Aug. 1967. Limiting the applicability ofthis method is the need of at least 8 inches of competent coreable rockfor a worthwhile measurement. Since a small of rock is measured in themethod, and because of the heterogeneity of rock weakness planes, theresults of extrapolating the data derived for the use of the results insolving full-size tunnel support problems, have been largelyunsuccessful.

Other known techniques for obtaining deformation measurements employclosure measuring instrumentation with which indications of the closureof the tunnel opening are made to appear beginning sometime after theexcavating system has progressed beyond the measuring site. Therefore,such techniques still do not record the initial deformation appearingwhen the excavation is carried on. Similar instrumenration havingutility in connection with deformation measuring equipment is describedin US. Pat. No. 3,108,716 entitled Control Mechanism" granted to L. A.Panek on Oct. 29, 1963. Discussed therein are changeable pressuremeasurements taken in rock, and the significance of these measurementsto possible rock deformation. Particularly noted is that control of rockdisplacement depends ultimately on controlling the pressure, and sincepressure creates displacement, knowledge of the direction and magnitudeof the rock pressure is likely to be more meaningful than knowledge ofthe direction of the rock displacement and deformation. Here again aswith the aforementioned borehole deformation gage, the measurements aremade in'a small borehole and reflect change in the boreholes immediatemedium only; whereas the present invention does not measure the changein a diameter of a borehole, but total radial displacement of the mediumaround any size adjacent opening. Moreover, because of the real timenature of the deformation data gained by the present invention, it is ofgreater importance to an understanding of tunneling and similarexcavating operations than deformation data derived by the priormethods.

SUMMARY OF THE INVENTION A prime objective of the invention is tomeasure the displacement of walls forming an underground opening whensuch opening is created, as in advancing the working face of a minetunnel. Outside the periphery of the proposed excavation and uniformlyspaced about the opening to be made, boreholes are drilled to a uniformdepth. A corresponding number of boreholes are drilled in stable rocksuch as may be found in adjacent sidewalls of the threshold tunnel.Rigid supports cemented in the further boreholes reference the positionsof mounting heads affixed to these supports. An anchor component isfixedly placed at the deep end of each of the peripheral boreholes, anda sensing probe mounted in each of the aforesaid heads extends into thecorresponding peripheral borehole and joins the fixed anchor in aflexible connection. The head thus effectively couples a sensing probeto a stable reference base whereby the probe is maintained in fixedposition out of contact with the borehole surface. An arrangement ofelectrical strain gages are affixed to a cantilever sensor structure atthe end of each probe, and the extended end of this cantilever isyieldingly held by the flexible connection. However, displacement of theanchors with the deformation, due to excavation in the proposed tunnel,bends the strain gages accordingly. Readouts from the gages to metercircuits provide data on which measurements of the extent and directionof the resulting deformations are based.

This and other objects and advantages of the invention will be moreclearly understood from the following description of a preferredembodiment of the invention considered together with the accompanyingdrawing wherein:

FIG. 1 is a plan view in cross section through a longitudinal centerline of an excavation site in a tunnel which is equipped with thepresent invention;

FIG. 2 is a cross-sectional view of one of the boreholes appearing inFIG. I as containing a probe structure and anchor, and revealing in acutaway representation details of a sensor assembly;

FIG. 2A presents a fragmentary showing of the sensor assembly of FIG. 2in which details of electrical parts and connections are schematicallyrepresented; and

FIG. 3 is a showing of the mounting head applied to couple a tubularsupport of the sensing probe to a referencing rigid beam.

Referring to FIG. 1, a first proposed excavation 10 is indicated asstarting at a working face 12 in an underground tunnel l4, and asculminating in an extension of tunnel l4 following a longitudinal axis16 thereof to a further working face 18 of a second proposed excavation20. Also appearing are side walls 22 and 24, and a floor 26, whichtogether with a roof (not shown), define the surfaces constituting thepreexisting tunnel l4 terminating at working face 12. In the embodimentdisclosed herein, six boreholes of like dimensions are uniformlyarranged around the tunnel extension to be completed by excavation 10,whereby these boreholes are aligned parallel to tunnel axis 16 spaced atapproximately a foot outside the periphery of the tunnel extension. Inaddition to the four boreholes 30, 32, 34 and 36 shown in FIG. 1, twofurther parallel boreholes (not shown) are disposed in the upper part ofthe tunnel. The walls defining preexisting tunnel 14 have six additionalboreholes which are placed radially disposed with respect to tunnel axisl6, and located about the tunnel in correspondence with and adjacent tothe aforesaid axially aligned boreholes. Also appearing in FIG. 1 areopenings 40, 42, 44 and 46 from which the radial boreholes extend intowalls 22 and 24, and floor 26, such that their longitudinal axes are atright angles to the longitudinal axes of the parallel boreholes whichpositionally correspond thereto. Two other radially disposed boreholes(not shown), are provided in the tunnel roof where they correspond tothe parallel boreholes near the roof in the manner heretofore explained.Although six boreholes of each disposition are designated for theinstant embodiment, as few as three boreholes of each type may be usedin connection with the invention.

Operationally associated with each pair of perpendicularly disposedboreholes is a deformation sensing apparatus. Since the sensingapparatus for the six borehole pairs are alike, an exemplary disclosureof one such apparatus follows with reference being made to the boreholepair including borehole 30. An anchor unit 50 is placed adjacent theinner or deep end of borehole 30, where it is partially situated a shortdistance into the working face 18. Referring now to FIG. 2, an expandedcylinder 52, capped on its ends by covers 54 and 56, will be seen asconstituting the anchor body. A length of pipe 58, supported in coveropenings, extends axially through cylinder 52 and has ends projectingoutside the covers. Within one of these ends is secured a caplike socket60 which constitutes part of a flexible connection 62 to be hereinaftermore fully described. The anchor shown herein as having utility in thepresent invention includes explosive deformation means whereby it isexpanded so as to be held by contact pressure on the surface of borehole30, as shown in FIG. 2. A further description of the anchor is given inUS. Pat. No. 3,40l,46l, entitled "Explosive Centerhole Anchor, andissued Sept. l7, l968 to E. W. Parsons et al.

As is hereinafter explained in detail, anchor 50 functions as anactuator for a strain sensing probe assembly 64 which in turn issteadily maintained in operative position by an elongated tubularelement 66. An extended end 68 of tubular element 66 projects fromborehole 30 along the longitudinal axis thereof. Through opening 40, andwithin a borehole 41 extending therefrom, is received an elongated roundbeam 70, which, as best seen in FIG. 1, is of such length as to spannearly the full length of borehole 41, and have an end portion 72thereof project outside the borehole. Beam 70 is rigidly fixed withrespect to wall 22 by sand and expandable cement 73 charged intoborehole 41 so as to become distributed about the surfaces of the beamand protuberances 74 thereon. Thus, the beam's projected end 72 isfixedly maintained directed along the longitudinal axis of borehole 41,which as hereinbefore indicated, is disposed perpendicular to thelongitudinal axis of borehole 30 and end 68 of tubular support element66.

Projected end 72 of beam 70 has attached thereto a generally rectangularcasing structure which constitutes an adjustable coupler 80 in that itserves to affix tubular element 66 to the beam in a requisite manner.Turning now to FIG. 3, it will be seen that beam end 72 terminates in athreaded portion 82 which engages threads of an opening 84 in an endplate 86 of coupler 80. Sideplates 88 and 90 and end plate 86, aresuitably joined together with conformable top and bottom pieces to makeup the coupler. Flanged clamps 92 and 94 are afiixed to coupler sides 88and 90, respectively, where the clamps collar parts 96 and 98 haveopenings located over slightly larger openings in the sides. Clamps 92and 94 are initially retained to pivot on coupler sides 88 and 90 byscrews 100 and 102 through the flanges whereby more precise alignment ofthe collar openings is made possible to facilitate the receipt thereinof tubular support 66. Further screws 104 to 107 are thereafter appliedthrough arcuate slots in the flanges to securely maintain the clamps inadjusted position by fastening into the coupler sides. An array ofthreaded radial openings through collar parts 96 and 98 receive thereinadjustable fasteners, such as bolts 110 to 113, which allow a finepositioning of the tubular element along the longitudinal axis ofborehole 30.

Strain sensing probe assembly 64 is supported on the inserted end oftubular element 66 so as to locate this assembly at a depth in borehole30 where a ball fitting 120 at the leading end of the assembly isreceived in socket 60 of fixed anchor 50. The aforesaid adjustments andalignments made possible by the structure of coupler 80, which properlyposition tubular element 66, shift this element about the pivotestablished in connection 62 by ball fitting 120 in socket 60. When thetubular element is thereby appropriately centered, it is out of contactwith the wall surfaces of borehole 30. More particularly, probe assembly64 is arranged between the tubular element and the anchored ball andsocket connection to properly situate a strain gage device 124 thereoffor operation in the borehole. A solid cylindrical base part 126 ofassembly 64 is fitted at one end thereofinto the front end opening oftubular element 66, and secured therein. A semicylindrical opposite endof base part 126 provides a plane surface 128. through the centerthereof, to which is fastened a cantilever sensor 130. A relativelynarrow flexible bar 132, fastened at one end thereof against base partsurface 128 by a holddown block 134 and traversing screws 136,constitutes a centrally disposed cantilever of the sensor. Ball fittingis attached by a stub block thereof to the extended end of bar 132 whereit functions as the aforesaid spherical support adapted to be bornewithin socket 60 of connection 62.

Areas on the top and bottom surfaces of bar 132, adjacent to itsattachment to surface 128, appear in FIG. 2A as having affixed theretostrain gage pairs 142 and 143, and 144 and 145, respectively. Straingages of any typical design, employing conductors of small cross sectionarranged parallel to the major dimension of bar 132, have utility in thepresent invention. A conventional bridge circuit, such as isschematically shown in FIG. 2A, is applicable to gain deformation datafrom signals derived in the aforesaid strain gages. A source of voltage148, and a readout meter 149 of this bridge circuit are shown in thefigure as connected to the strain gages by way of leads 150 and 151, andleads 154 and 155, respectively. Leads 150 and 151 have theirconnections completed to gages 142 and 144, and gages 142 and 145,respectively, by way of leads and 161, and leads 166 and 167,respectively. Leads 154 and 155 have their connections completed togages 143 and 145, and gages 143 and 144, respectively, by way of leads162 and 163, and leads 164 and 165, respectively. The leads connectingremotely located voltage source 148 and meter 149 with the gagesactually extend through a passage and probe base part 126, as indicatedby dashed line and collars 171, and thence through tubular element 66and a coupling connection 172. Conventional terminals affixed in basepart surface 128, as exemplified by posts 173, serve to effect theimmediate connections between source and meter leads and the strain gageleads. Sensing probe assembly 64 is completed by a short tubular sleevefitted over and secured to a portion of base part 126 whereby it extendsover the gages and a part of cantilever 132 to constitute a protectivecover for sensor device 124.

Set out hereinbelow is a procedure for measuring initial and timedependent deformation of the medium surrounding a tunnel-typeunderground opening as it is being excavated and a period following,respectively, according to the present invention. In connectiontherewith reference is made to the structural embodiment previouslydescribed.

On the surface ofthe wall encircling the area through which excanationproceeds, or working face 12, and at a short distance from theexcavation proper, uniformly spaced marks are made to designate pointswhereat boreholes are to be drilled parallel to the longitudinal axis 16of the proposed passage or tunnel. Stable walls, represented in FIG. 1by surfaces 22 and 24 ofa preexisting tunnel leading to the proposedpassage and located adjacent to the encircling surface, arecorrespondingly marked for boreholes to be drilled radially disposedwith respect to the aforesaid longitudinal axis, so that their axes areperpendicular to the parallel boreholes. After the aforesaid radialboreholes 41 are drilled, each receives a rigid tube or beam 70 which iscemented therein with an end part 72 thereof extending outside theborehole. An adjustable coupler head 80 is centrally disposed andsecured on the ex tended end of the beam part. The parallel boreholes 30are then drilled, and an anchor 50, equipped with one part 60 of aflexible connector 62, is solidly affixed at the inner end of each ofthe parallel boreholes with its connector part facing out.

A sensing probe arrangement, including a sensor assembly 64 attached tothe end of an elongated support 66, is inserted through adjustableopenings in each of the coupler heads so as to extend from this mounttherefor and into a parallel borehole corresponding thereto. Acantilever sensor 124 of each of the aforesaid assemblies carries acomplementary connector part 120 for the flexible connection 62 whichfacilitates this connection as the probe arrangement is placed in theparallel borehole and adjusted in and by the coupler head openings tosituate the probe arrangement so that no part thereof comes into contactwith the borehole surface. Electrical strain gages arranged on eachcantilever sensor of the respective assemblies have circuit leadsextending back through supporting structure of the probe to reach amonitoring station at a relatively remote point. The calibrating of suchstrain gages is disclosed in the U.S. Bureau of Mines Report ofinvestigations 5978, entitled Borehole Deformation Gage for DeterminingThe Stresses ln Mine Rock," by L. A. Obert, R. H. Merrill, and T. A.Morgan, dated May 1962. Also demonstrated is that with knowledge of rockmedium elastic constants consideration of borehole deformationmeasurements facilitates the calculation of direction and magnitude ofthe principal stresses in the rock medium.

The procedure set out accomplishes referencing of points in the wall ofa future opening, such as working face 18 in a plane defined by theanchors, to steadied points established by rigid beams cemented in astable wall. Consequently, as excavation l0 progresses and the wallsadjacent the opening move, the anchors made integral therewith alsomove, whereas the stabilized probe support structures do not move sincethey do not touch the rock of the nearby surrounding medium. However,movement of the anchor bends the cantilevers of the sensor assemblieswhich are otherwise held rigid with respect to the stable walls. When acantilever lengthens or shortens upon bending, the strain gages thereoncause a change in electrical resistance which through a calibratedreadout instrument at the monitoring station can be interpreted to showexact movement of a tunnel wall during excavation. These measurementsand rate of movement data can be combined with other known rock propertydata and used to produce information on rock anisotropy, and in siturock stress, including, for example, the determination of thedistribution in a plane of stress around underground openings, ground inproblem areas, and artificial supports. Also made possible is continuousrecording of rock deformation around an underground opening as theopening is being excavated. Use of the described procedure and systempermits total radial displacement of the wall rock around an excavationas it is created to be measured to an accuracy of 20.00005 inch. As iswell understood in the art, the magnitude and direction of rockdeformation are functions of interrelated parameters including in situstress field, the opening geometry, the physical properties of the rockand geologi cal features such as fracture orientation, and frequency andcohesiveness of the rock. Thus, measured displacements of the rockmedium as hereinbefore described, indicate the effect of the foregoingcauses of the deformation. Further, measurements taken in accordancewith the present invention at the fullscale opening as it is excavated,reflect the effects of gross rock features in contrast to those of theprior art where only localized features are obtained when diametricalchange in small boreholes is measured. Consequently, measured andderived data and information provided by use of the invention allows arequisite understanding of the fitness or competence of the rock mediumfor pursuing therein an excavation extending an underground opening intoa tunnel.

Other means of supporting the sensing probe arrangement may be employedfor the procedure as long as provision is made for a stable medium towhich the probe reference structure can be fixed. Thus, the boreholesalongside the proposed excavation might be extended, and the sensingprobes placed in the holes with their reference support ends firstwhereby such ends can be cemented at the deepest points of the holes.With the remainder of the probes supported so that they do not touch therock of their holes, the anchors could be in serted in the holes andpositioned at the points where information on the ground movement isdesired. Other transducers than strain gages can be used. A linearvariable differential transformer can be attached to the sensing probearm. The differential transformer could be attached to the wall of aborehole and the core attached to the sensing probe. Movement of thetransformer by rock shift gives rise to electrical output proportionedto its displacement in relation to the core. A similar arrangement couldbe made using a potentiometer for the transducer. instead of using afixed anchor 50 (either explosive type or cemented in hole) a post maybe fixed to cantilever 132 replacing the ball fitting 120. The postextends to the borehole wall facilitating a measure of the displacementof that point. However none of these alternatives are as stable as thestrain gage transducer described in the preferred embodiment since theywould be more subject to vibrations not originating with the rock beingmonitored.

While a preferred embodiment and procedure for the invention has beendescribed and illustrated, it is to be understood that the invention isnot limited thereby but is susceptible to changes in form and detail.

I claim:

1. A method for measuring deformation in walls of a tunnel duringexcavation thereof from a working face at an end of a preexistingpassage leading thereto comprising:

drilling a first plurality of substantially aligned holes to extend fromthe openings thereof spaded around in an area on said working facesurrounding the periphery of the opening said excavation is to make insaid working face approximately perpendicularly to said working face;

drilling in areas of stable walls forming said preexisting passage,which are adjacent to said working face, a second plurality of holeshaving openings, respectively spaced around in correspondence with saidfirst hole openings;

constituting contiguous to the deep end of each of said first holes ananchor supporting an articular connective element, and insertingthereafter, so as to join with said anchor at said connective element, astrain data sensing probe having an end extending outside said holewhereby the longitudinal axes of said probes cross extensions oflongitudinal axes of said second holes;

affixing in each said second hole, substantially along said axisthereof, a beam having an end thereof extending outside said hole anddisposed adjacent said outside probe extension;

coupling each said adjacent beam and sensing probe ends outside theirrespective holes whereby each sensing probe is maintained stabilizedwith respect to said walls of said preexisting passage, and centered insaid working face holes out of contact with the walls thereof, and

reading signal outputs of said plurality of sensing probes as saidexcavation progresses to form a tunnel extension of said preexistingpassage whereby the derivation of rock competence information on saidtunnel walls is enabled and wherefrom support problems of subsequentexcavations to further extend said tunnel walls can be anticipated.

2. The method of claim 1 wherein said first and second pluralities ofhole openings are uniformly spaced apart in first and second planes,respectively, whereby overall measurements of radial displacements ofthe wall rock around an excavation as it is created are facilitated.

3. An apparatus for measuring deformation in the medium defining atunnel being excavated employing a plurality of deformation measuringmechanisms separately located in im dividual holes through said mediumspaced around said excavation, each said mechanism comprising:

an individual anchor affixed at the internal end of each one of saidindividual holes and each said anchor having a fixed connectivecomponent as an integral part thereof;

a sensing probe situated in each one of said individual holes,

said probe including a deformation sensing head, a tubular supportelement having an end inserted into said individual hole correspondingto said probe comprising said support element, and a part extendingthrough and outside said corresponding individual hole, said sensinghead being afiixed at one end to said inserted end of said supportelement and having affixed on another end an opera tional element, saidoperational element having secured thereto, adjacent said another end ofsaid sensing head, an arrangement of electrical strain gages, andattached to said operational element, at an end thereof extended towardsaid fixed connective component, a further connective componentcomplementarily fitting together with said connective element;

a means retained substantially rigid with respect to a stable surfaceadjacent said tunnel excavation working face, having attached thereto acoupling means wherein is securely attached said tubular support elementwhereby said sensing probe is maintained centered in said hole and outof contact with the surfaces thereof; and

a means to read out strain data signals generated by said strain gagesduring said excavation.

4. The apparatus of claim 3, wherein said anchor comprises a metallicenclosure containing an explosive adapted to be detonated in said holewhereby a resulting expansion of said enclosure forces walls thereofupon surfaces inside said hole 5, The apparatus of claim 3, wherein saidsensing head comprises a cylindrical mount partially fitted and securedwithin said tubular support element, and the extension thereof outsidesaid tubular element having a carrier surface to which is secured saidone end of said operational element and said electrical strain gagesbeing symmetrically disposed on opposite surfaces of said operatingelement.

6. The apparatus of claim 5 wherein said operational element is a flatnarrow strip constituting a cantilever having said one end thereof fixedto said cylindrical mount carrier surface, and the other end thereofsupported by said connective components.

7, The apparatus of claim 3 wherein said further and fixed connectivecomponents constitute a ball and socket link between said anchor andsaid sensing probe.

8. The apparatus of claim 3 wherein said means retained substantiallyrigid with respect to a stable surface is an elongated beam having amajor part thereof made integral with the medium constituting saidsurface and the extended end thereof outside said medium, said couplingmeans is a casing having aligned openings in parallel walls thereof anda further opening in a wall at right angles to said parallel walls, saidextended end of said beam having means rigidly securing said end intosaid further casing wall, an adjustable plate operative on each of saidparallel walls and adapted to be fixed in adjusted position, each saidplate having a collar defining an opening therein alignable with saidwall openings at said adjusted position, said tubular support elementoutside said hole having parts residing in said wall openings andcollars, and adapted to be rigidly attached to said casing by fasteningsin said collars.

1. A method for measuring deformation in walls of a tunnel duringexcavation thereof from a working face at an end of a preexistingpassage leading thereto comprising: drilling a first plurality ofsubstantially aligned holes to extend from the openings thereof spadedaround in an area on said working face surrounding the periphery of theopening said excavation is to make in said working face approximatelyperpendicularly to said working face; drilling in areas of stable wallsforming said preexisting passage, which are adjacent to said workingface, a second plurality of holes having openings, respectively spacedaround in correspondence with said first hole openings; constitutingcontiguous to the deep end of each of said first holes an anchorsupporting an articular connective element, and inserting thereafter, soas to join with said anchor at said connective element, a strain datasensing probe having an end extending outside said hole whereby thelongitudinal axes of said probes cross extensions of longitudinal axesof said second holes; affixing in each said second hole, substantiallyalong said axis thereof, a beam having an end thereof extending outsidesaid hole and disposed adjacent said outside probe extension; couplingeach said adjacent beam and sensing probe ends outside their respectiveholes whereby each sensing probe is maintained stabilized with respectto said walls of said preexisting passage, and centered in said workingface holes out of contact with the walls thereof, and reading signaloutputs of said plurality of sensing probes as said excavationprogresses to form a tunnel extension of said preexisting passagewhereby the derivation of rock competence information on said tunnelwalls is enabled and wherefrom support problems of subsequentexcavations to further extend said tunnel walls can be anticipated. 2.The method of claim 1 wherein said first and second pluralities of holeopenings are uniformly spaced apart in first and second planes,respectively, whereby overall measurements of radial displacements ofthe wall rock around an excavation as it is created are facilitated. 3.An apparatus for measuring deformation in the medium defining a tunnelbeing excavated employing a plurality of deformation measuringmechanisms separately located in individual holes through said mediumspaced around said excavation, each said mechanism comprising: anindividual anchor affixed at the internal end of each one of saidindividual holes and each said anchor having a fixed connectivecomponent as an integral part thereof; a sensing probe situated in eachone of said individual holes, said probe including a deformation sensinghead, a tubular support element having an end inserted into saidindividual hole corresponding to said probe comprising said supportelement, and a part extending through and outside said correspondingindividual hole, said sensing head being affixed at one end to saidinserted end of said support element and having affixed on another endan operational element, said operational element having secured thereto,adjacent said another end of said sensing head, an arrangement ofelectrical strain gages, and attached to said operational element, at anend thereof extended toward said fixed connective component, a furtherconnective component complementarily fitting together with saidconnective element; a means retained substantially rigid with respect toa stable surface adjacent said tunnel excavation working face, havingattached thereto a coupling means wherein is securely attached saidtubular support element whereby said sensing probe is maintainedcentered in said hole and out of contact with the surfaces thereof; anda means to read out strain data signals generated by said strain gagesduring said excavation.
 4. The apparatus of claim 3, wherein said aNchorcomprises a metallic enclosure containing an explosive adapted to bedetonated in said hole whereby a resulting expansion of said enclosureforces walls thereof upon surfaces inside said hole.
 5. The apparatus ofclaim 3, wherein said sensing head comprises a cylindrical mountpartially fitted and secured within said tubular support element, andthe extension thereof outside said tubular element having a carriersurface to which is secured said one end of said operational element andsaid electrical strain gages being symmetrically disposed on oppositesurfaces of said operating element.
 6. The apparatus of claim 5 whereinsaid operational element is a flat narrow strip constituting acantilever having said one end thereof fixed to said cylindrical mountcarrier surface, and the other end thereof supported by said connectivecomponents.
 7. The apparatus of claim 3 wherein said further and fixedconnective components constitute a ball and socket link between saidanchor and said sensing probe.
 8. The apparatus of claim 3 wherein saidmeans retained substantially rigid with respect to a stable surface isan elongated beam having a major part thereof made integral with themedium constituting said surface and the extended end thereof outsidesaid medium, said coupling means is a casing having aligned openings inparallel walls thereof and a further opening in a wall at right anglesto said parallel walls, said extended end of said beam having meansrigidly securing said end into said further casing wall, an adjustableplate operative on each of said parallel walls and adapted to be fixedin adjusted position, each said plate having a collar defining anopening therein alignable with said wall openings at said adjustedposition, said tubular support element outside said hole having partsresiding in said wall openings and collars, and adapted to be rigidlyattached to said casing by fastenings in said collars.